Technical Field
Embodiments of the present disclosure relate to transimpedance amplifiers.
Description of the Related Art
Transimpedance amplifiers are well known in the art. Generally, a transimpedance amplifier (TIA) is a current-to-voltage converter, which usually also performs an amplification. For example, such TIAs are used, e.g., in optical receivers in order to convert the current provided by a photodiode into a corresponding voltage signal. Accordingly, a transimpedance amplifier should exhibit a low impedance to the photodiode and isolate it from the output voltage of the amplifier.
FIG. 1 shows in this respect a typical optical transmission system, which comprises an optical transmitter circuit 1 and an optical receiver circuit 3.
In the example considered, the transmitter circuit 1 comprises a signal generator 10 and optical transmitter 12 for generating an optical waveform, such as an LED (light emitting diode) or a laser diode. Substantially, the signal generator 10 receives a digital or analog data signal DI at input and generates a drive signal for the optical transmitter 12 as a function of the data signal DI, thereby transmitting the data signal DI via a modulation of the light emitted by the optical transmitter 12.
The optical receiver circuit 3 comprises a light sensor 30, such as a photodiode PD, a transimpedance amplifier 32 and a processing circuit 36.
In the example considered, the optical transmitter 12 may be coupled to the light sensor 30 by means of an optical fiber 2, and generally the light sensor 30 is configured to receive the light generated by the optical transmitter 12 (taking into account possible losses and noise generated by the fiber 2).
Specifically, in the example considered, the transimpedance amplifier 32 converts the current provided by the photodiode PD into a corresponding voltage signal Vout indicative of the intensity of light received by the photodiode PD.
Accordingly, the processing circuit 36, which generally may be an analog and/or digital circuit, such as a micro-processor, e.g., a DSP (digital signal processor), may elaborate the voltage signal Vout in order to detect the data signal DI.
Generally, between the transimpedance amplifier 32 and the processing circuit 36 may be provided also further analog and/or digital signal processing stages 34, such as one or more amplifier stages and/or filters, such as bandpass filters.
FIG. 2 shows in this respect possible implementations of the optical frontend of the receiver circuit 3.
Specifically, in the example considered, the transimpedance amplifier 32 is based on a npn bipolar transistor Q1 having a given transconductance gm.
Specifically, in the example considered, the base of the transistor Q1 is connected to the photodiode PD, the collector is connected to a constant positive supply voltage, such as VDD (e.g., a voltage between 1 and 5 VDC, with respect to ground GND), by means of a first resistor RC, and the emitter is connect to ground GND by means of a second resistor RE. In particular, in the example considered, the cathode of the photodiode PD is connected to the base of the transistor Q1.
In typical applications the photodiode PD is biased in some way. For example, in FIG. 2 the anode of the photodiode PD is connected (e.g., directly) to ground GND and the cathode of the photodiode PD is connected via a resistor or an active impedance Rbias to a positive (preferably constant) bias voltage Vbias, which could also be the supply voltage VDD. Accordingly, the transistor Q1 is used basically in a common emitter configuration and the output voltage corresponds to the voltage at the collector of the transistor Q1.
In particular, as shown in FIG. 3, the photodiode PD may be modelled as an ideal photodiode 100, i.e., a current generator generating a current IS, having connected in parallel a capacitor CPD and a resistor RPD.
Accordingly, the current provided by the photodiode 100 will generate a variation at the base of the transistor Q, which will be amplified by the transistor Q1. Accordingly, in the examples considered, the voltage Vout will reflect the variations of the current IS provided by the photodiode PD and thus will be indicative for the intensity of light received by the photodiode PD.
As shown in FIG. 4, a substantially similar schematic may be used also by replacing the bipolar transistor Q1 with a FET (field effect transistor), such as a MOSFET (metal-oxide-semiconductor field-effect transistor) Q2 by replacing the resistors RC and RE with corresponding resistors RD and RS at the drain and source of the transistor Q2 and connecting the photodiode PD to the gate of the transistor Q2.
In order to improve the bandwidth of the optical frontend, different techniques have been proposed.
For example, FIG. 5 shows the so called shunt inductive peaking.
Specifically, in this technique, an inductor LP is connected in series with the resistor RC in order to reduce the influence of the output capacitance Cout of the optical front end, i.e., the capacitance between the output Vout and ground GND.
Accordingly, this inductor LP will generate a resonance with the output capacitance Cout, thereby reducing the low pass filter effect of the capacitance Cout.
Conversely, FIG. 6 shows the so called series inductive peaking.
Specifically, in this technique, an inductor LS is connected between the cathode of the photodiode PD and the base of the transistor Q1. Accordingly, this inductor LS may be used to reduce the influence of the capacitance CPD (and possible other capacitors connected in parallel with the photodiode PD) at the input of the transimpedance amplifier 32.
Generally, the previous techniques, i.e., shunt inductive peaking and series inductive peaking, may also be combined.
The inventors have observed that the above techniques may not be sufficient.
Specifically, as shown in FIG. 7, indeed also the transimpedance amplifier 32 exhibits an input capacitance CBE, which, e.g., corresponds to the base-emitter capacitance of the bipolar transistor Q1.
Accordingly, the capacitances CPD/CBE and the inductor LS form indeed a CLC filter structure, which still limits the bandwidth of the optical front end.